In Ellson U.S. Pat. No. Re. 30,692 reissued Jul. 28, 1981 (reissue of U.S. Pat. No. 4,185,282 issued Jan. 29, 1980) entitled "Method And Apparatus For Measuring Telephone Line Characteristics" there is described a method and apparatus for deriving parameters of a telephone line from current and charge flow measurements taken during three successive voltage states applied to the tip and ring wires of the line via series resistors of known resistance. The parameters include, in particular, resistances, capacitances, and foreign voltages of the line.
Due to the characteristics of the line, each of the successive voltage states produces transient voltages. For sufficiently accurate measurement results, final value or steady state voltage measurements are required, and these have traditionally been performed after the transient voltages have died, for example in the Ellson patent by waiting for two successive identical measurement samples. Although the Ellson patent indicates that a test period may typically be less than three seconds, it recognizes that the measurement time is directly related to the leakage resistance and the capacitance of the telephone loop.
The time required for a transient to die may in fact be as long as 500 seconds (5 times the time constant of a line with a loop capacitance of 10 .mu.F with a resistance to ground of 10M.OMEGA.). A telephone operating company may wish to measure the parameters of all telephone lines on a nightly basis in order to detect potential problems as quickly as possible, with a consequent need for high speed measurements, but this is not practical with such long waiting times to allow transients to die so that steady state measurements can be made as in the Ellson patent.
In an attempt to avoid this difficulty, it is known to reduce the time constant of the line by driving it with a low impedance, but this can result in inaccurate measurement results. Inaccurate results arc also produced if measurements arc performed before the transients have completely died, so that the voltage measurements used arc not the final value or steady state voltages.
An object of this invention is to provide an improved method of monitoring an electrical circuit, such as a two-wire telephone line, including resistive and capacitive characteristics.